In general, the intensity of light in a transmissive media decays exponentially. Over a round-trip path, the intensity of a beam of light is attenuated by e−2cr where c is the total attenuation coefficient of the medium, and r is the distance from source to target. The distance at which light is attenuated by e−1 is referred to as an attenuation length (AL). In clear water this distance is between 20 to 30 meters, and even less in a turbid media such as ocean water, clouds, fog, smoke or tissue. This places practical limits on underwater optical imaging. At distances beyond about three AL, laser-based systems are typically used to image underwater targets. The U.S. Navy has been continuously developing underwater imaging systems for use in a variety of applications including Autonomous Underwater Vehicles (AUVs) and Remotely Operated underwater Vehicles (ROVs) to provide navigational data as well as other data for detecting the presence of mines and AUV's and ROV's.
Imaging theory based on the Modulation Transfer Function (MTF) predicts that an ideal self-luminous source can be detected well over 15 AL under nighttime observation, depending on the source size. In order to attempt to reach this theoretical limit, the engineering task therefore becomes one of designing a system that can extract target detail as a sequence of discrete self-luminous sources. One way to do this is to bring a laser source as close to a target as possible. However, a nearby laser source that illuminates the entire target at the same time is not the answer. Scattering due to particles along the path between the target and a distant receiver (a separate unit from the laser source) will mix photons between neighboring pixels together and the target will become unrecognizable after a half-dozen attenuation lengths. Under these circumstances, a better imaging system, especially for turbid media, is one that makes use of an “illuminator” which scans each of the target “pixels” in a predetermined sequence as closely as possible to the target. The scanning of the target will produce a time-varying intensity (TVI) signal at the distant receiver that is not adversely affected by scattering. This is due to the fact that since the laser illuminates only a small portion of the target of interest at a time, all of the light that is reflected by the scene at each scan position—even the multiply scattered light—carries “useable” information about the target. Thus, the receiver can collect all of the light reflected by each pixel in the scene and still produce high quality images over many attenuation lengths. An image of the target and its details can then be reconstructed remotely (e.g., onboard a nearby ship) as long as the predetermined scanning sequence used for target illumination is known.
Such a system was built in the early 1970's at the Scripp's Visibility Laboratory and experimental data collected by this prototype system confirmed the soundness of the approach and an associated imaging capability of between 15 and 20 attenuation lengths at 640 nm. See, S, Q, Duntley, R. W. Austin, R. L. Ensminger, T. J. Petzold, and R C. Smith, “Experimental TVI System Report,” Visibility Laboratory Technical Report 74-1, Part I, July, 1974 and Part II, October 1974. A flash lamp co-located with the laser source produced an optical signal that was used for synchronization. The flash of light indicated the beginning of a scan, and the remote receiver synchronized its data collection with this optical trigger. This system produced impressive underwater images at greater than 20 attenuation lengths in turbid harbor water.
A drawback of this system was that a separate flash lamp source was needed to convey the start of a scan to the distant receiver. Moreover, this was the only information that the receiver had concerning the scan. Therefore, certain things had to be assumed by the receiver, such as standoff distance, scan rate, and scan angle, so that it could correctly recreate the image. Another disadvantage was the fact that this initial system used a red laser with a wavelength of 640 nm. Better performance is generally expected with a laser in the blue-green region of the optical spectrum due to the lower absorption at shorter wavelengths. Finally, two optical receivers were needed in order to separate the laser light reflected from the scene of interest from the flash lamp used to synchronize the beginning of the scan. Embodiments according to the present invention are directed to solving the foregoing problems.